1. Field of the Invention
The present invention relates to a power amplifier applied for example to a transmitter of a wireless communication system, more particularly relates to an improvement of a bias circuit setting the bias current.
2. Description of the Related Art
The bias current of a power amplifier is a very important factor determining its characteristics. Especially, power amplifiers for transmission used for digital mobile phones etc. require high linearity, so the setting of the bias current is becoming extremely important.
FIG. 1 is a circuit diagram of a general bias circuit of a power amplifier disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-120414.
As shown in FIG. 1, the bias circuit has a field effect transmitter (FET) 1, a bias voltage supply terminal 2 of a gate bias voltage Vgg, a resistance element R1, and a resistance element R2. In the bias circuit of FIG. 1, the resistance element R1 and the resistance element R2 are connected in series between the bias voltage supply terminal 2 and the ground GND, and the connection point of the resistance element R1 and R2 is connected to a gate terminal G of the FET 1. A drain terminal D of the FET 1 is connected to a supply line of a power source voltage Vdd, and a source terminal S of the FET 1 is grounded. The gate bias voltage Vgg is supplied from the bias voltage supply terminal 2 to set the bias current.
Summarizing the problems to be solved by the invention, the above power amplifier does not suffer from any problems at room temperature, but if the ambient temperature changes, there is the problem that the bias current of the power amplifier changes greatly from the setting at room temperature especially at low temperature or high temperature. As a result, the linearity of the power amplifier remarkably deteriorates.